Grid legend: are there any rules around it?

enter image description hereI have a grid, and I'm thinking about adding a legend to it. Like there are legends for charts.

Here's what would go in the legend:

  1. 70% of columns are editable. Known pattern would be to put a pencil icon in each column. But I end up with an army of pencils...:)

  2. Some line items are missing data. I need to label them, so user knows.

  3. There is one column where content can be appended; not edited.

I thought of color-coding this 3 things. But, come to think of it, I need a non-color reliant way to communicate this info., for ADA purposes. So, I end up with icons, anyway...

Right? Any ideas?

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